Optical glass, preform, and optical element

ABSTRACT

An optical glass having a small partial dispersion ratio (θg,F), while having a refractive index (nd) and Abbe number (νd) within desired ranges, is obtained. The optical glass, in mass %, comprises 10.0 to 70.0% of an SiO2 component, 1.0 to 50.0% of an Nb2O5 component, and 1.0 to 30.0% of an Na2O component, and has a refractive index (nd) of 1.62 to 1.75, an Abbe number (νd) of 30 to 42, and a partial dispersion ratio (θg,F) of no greater than 0.594.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 U.S.C. § 119 to Japanese Application Nos. 2015-221565, 2015-221566, 2015-221567, each filed Nov. 11, 2015; and Japanese Application Nos. 2016-213163, 2016-213164 and 2016-213165, each filed Oct. 31, 2016, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an optical glass, a preform, and an optical element.

Related Art

Optical systems such as digital cameras and video cameras and the like, to varying degrees have blurring referred to as aberrations. These aberrations are classified as monochromatic aberrations and chromatic aberrations, and in particular, chromatic aberrations are strongly dependent on the material characteristics of the lenses used in the optical system.

Generally, chromatic aberrations are corrected by combining low dispersion convex lenses with high dispersion concave lenses, but these combinations can only correct aberrations in the red region and green region, while aberrations in the blue region remain. These aberrations in the blue region which cannot be eliminated are referred to as secondary spectra. In order to correct these secondary spectra, it is necessary to carry out optical design which takes into account shifts in the g-line (435.835 nm) of the blue region. At this time, as an indicator of the optical characteristics which are noted during the optical design, the partial dispersion ratio (θg,F) is used. In the above described optical system combining low dispersion lenses and high dispersion lenses, by using an optical material having a large partial dispersion ratio (θg,F) at the low dispersion lens side, and an optical material having a small partial dispersion ratio (θg,F) at the high dispersion lens side, secondary spectra can be well corrected.

The partial dispersion ratio (θg,F) is shown by the below formula (1). θg,F=(ng−nF)/(nF−nc)  (1)

In optical glasses, there is an approximately linear relationship between the partial dispersion ratio (θg,F) which shows the partial dispersion properties in the short wavelength range, and the Abbe number (νd). The straight line which shows this relationship, shown by a straight line connecting two points when plotting the partial dispersion ratio and the Abbe number of NSL7 and PBM2 in a Cartesian coordinate system where the partial dispersion ratio (θg,F) is the ordinate and the Abbe number (νd) is the abscissa, is referred to as the normal line (refer to FIG. 1). Normal glasses which are the standards of the normal line differ among different optical glass manufacturers, but this is defined by each manufacturer according to approximately equal slopes and intercepts. (NSL7 and PBM2 are optical glasses manufactured by Ohara, Inc., and PBM2 has an Abbe number (νd) of 36.3 and a partial dispersion ratio (θg,F) of 0.5828, and NSL7 has an Abbe number (νd) of 60.5 and a partial dispersion ratio (θg,F) of 0.5436.

Herein, as glasses having an Abbe number (νd) of 30 to 42, for example, the optical glasses according to Patent Documents 1 and 2 are known.

Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2002-029777

Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2008-239478

SUMMARY OF THE INVENTION

However, the glass disclosed in Patent Document 1 has a partial dispersion ratio which is not small, and is not sufficient for use as a lens for correcting the above mentioned secondary spectra. Further, the glass disclosed in Patent Document 2, while having a relatively small partial dispersion ratio, has a large Abbe number, and there is demand for a glass with a smaller Abbe number.

The present invention was made in consideration of the above described problems, and has the objective of providing an optical glass with a small partial dispersion ratio (θg,F) while having a refractive index (nd) and an Abbe number (νd) within the desired ranges.

The present inventors, as a result of repeated diligent experimentation and research to solve the above described problems, discovered that a glass having a low partial dispersion ratio, and a high refractive index and low Abbe number (high dispersion) within the desired ranges can be provided by a glass comprising an SiO₂ component and an Nb₂O₅ component, and thus completed the present invention.

Specifically, the present invention provides the following.

The first aspect of the present invention is an optical glass comprising, in mass %,

10.0 to 70.0% of an SiO₂ component,

1.0 to 50.0% of an Nb₂O₅ component,

1.0 to 30.0% of an Na₂O component,

and having

a refractive index (nd) of 1.62 to 1.75,

an Abbe number (νd) of 30 to 42, and

a partial dispersion ratio (θg,F) of no greater than 0.594.

The second aspect of the present invention is an optical glass according to the first aspect, wherein, in mass %, a content of a B₂O₃ component is no greater than 25.0%.

The third aspect of the present invention is an optical glass according to the first or second aspect, wherein a mass ratio of (Li₂O+Na₂O)/(ZrO₂) is no less than 0.50.

The fourth aspect of the present invention is an optical glass according to any one of the first to third aspects, wherein, in mass %, a content of an Li₂O component is no greater than 20.0%.

The fifth aspect of the present invention is an optical glass according to any one of the first to fourth aspects, wherein a mass ratio (SiO₂)/(SiO₂+B₂O₃) is no less than 0.50.

The sixth aspect of the present invention is an optical glass according to any one of the first to fifth aspects, wherein a mass ratio (SiO₂)/(SiO₂+B₂O₃) is no greater than 0.95.

The seventh aspect of the present invention is an optical glass according to any one of the first to sixth aspects, wherein, in mass %, a content of a ZrO₂ component is no greater than 25.0%

The eighth aspect of the present invention is an optical glass according to any one of the first to seventh aspects, wherein, in mass %,

a K₂O component is 0 to 20.0%,

a TiO₂ component is 0 to 20.0%,

an MgO component is 0 to 10.0%,

a CaO component is 0 to 10.0%,

an SrO component is 0 to 10.0%,

a BaO component is 0 to 20.0%,

a TaO₂ component is 0 to 10.0%,

an La₂O₃ component is 0 to 10.0%,

a Gd₂O₃ component is 0 to 10.0%,

a Yb₂O₃ component is 0 to 10.0%,

a Y₂O₃ component is 0 to 20.0%,

a P₂O₅ component is 0 to 10.0%,

a GeO₂ component is 0 to 10.0%,

an Al₂O₃ component is 0 to 15.0%,

a Ga₂O₃ component is 0 to 10.0%,

a WO₃ component is 0 to 10.0%,

a Bi₂O₃ component is 0 to 10.0%,

a ZnO component is 0 to 30.0%,

a TeO₂ component is 0 to 15.0%,

an SnO₂ component is 0 to 5.0%, and

an Sb₂O₃ component is 0 to 1.0%.

The ninth aspect of the present invention is an optical glass according to any one of the first to eighth aspects, wherein a mass sum of an Rn₂O component (in the formula, Rn is at least one selected from the group consisting of Li, Na, and K) is 1.0% to 30.0%.

The tenth aspect of the present invention is an optical glass according to any one of the first to ninth aspects, wherein a mass ratio of Li₂O/Rn₂O is no less than 0.01.

The eleventh aspect of the present invention is an optical glass according to any one of the first to tenth aspects, wherein a mass sum of an RO component (in the formula, R is at least one selected from the group consisting of Mg, Ca, Sr, and Ba) is no more than 25.0%.

The twelfth aspect of the present invention is an optical glass according to any one of the first to eleventh aspects, wherein a mass sum of an Ln₂O₃ component (in the formula, Ln is at least one selected from the group consisting of Y, La, Gd, and Yb) is no more than 20.0%.

The thirteenth aspect of the present invention is a preform for use in a polishing process or precision press molding, consisting of the optical glass of any one of the first to twelfth aspects.

The fourteenth aspect of the present invention is an optical element consisting of the optical glass according to any one of the first to twelfth aspects.

According to the present invention, it is possible to obtain an optical glass having a small partial dispersion ratio (θg,F), while having a refractive index (nd) and Abbe number (νd) within the desired ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing the normal line in Cartesian coordinates with the partial dispersion ratio (θg,F) as the ordinate and the Abbe number (νd) as the abscissa.

FIG. 2 is a drawing showing the relationship between the partial dispersion ratio (θg,F) and the Abbe number (νd) for examples of the present invention.

FIG. 3 is a drawing showing the relationship between the refractive index (nd) and the Abbe number (νd) for examples of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The optical glass of the present invention comprises, in mass %, an SiO₂ component of 10.0 to 70.0%, an Nb₂O₅ component of 1.0 to 50.0%, and an Na₂O component of 1.0 to 30.0%, and has a refractive index (nd) of 1.62 to 1.75, an Abbe number (νd) of 30 to 42, and a partial dispersion ratio (θg,F) of no greater than 0.594.

In a glass comprising an SiO₂ component and an Nb₂O₅ component, it is possible to obtain a glass having a low partial dispersion ratio, as well as a high refractive index and a low Abbe number (high dispersion) within the desired ranges.

Among these, a first optical glass comprises, in mass %, a SiO₂ component of 10.0 to 70.0%, an Nb₂O₅ component of 1.0 to 50.0%, and an Na₂O component of 1.0 to 30.0%, wherein a content of a B₂O₃ component is no greater than 20.0%, and has a refractive index (nd) of 1.62 to 1.75, an Abbe number (νd) of 30 to 42, and a partial dispersion ratio (θg,F) of no greater than 0.594.

In a glass comprising an SiO₂ component and an Nb₂O₅ component, even if a content of a B₂O₃ component has been reduced, a glass having a high refractive index and low Abbe number (high dispersion) within the desired ranges, and a low partial dispersion ratio can be obtained.

Further, a second optical glass comprises, in mass %, an SiO₂ component of 10.0 to 70.0%, an Nb₂O₅ component of 1.0 to 50.0%, and an Na₂O component of 1.0 to 25.0%, and has a mass ratio of (Li₂O+Na₂O)/(ZrO₂) of no less than 0.50, a refractive index (nd) of 1.62 to 1.70, an Abbe number (νd) of 31 to 42, and a partial dispersion ratio (θg,F) of no greater than 0.590.

In a glass comprising an SiO2 component and an Nb₂O₅ component, in particular even in the case of comprising an Na₂O component, and moreover, having a large mass ratio of (Li₂O+Na₂O)/(ZrO₂), a glass having a high refractive index and low Abbe number (high dispersion) within the desired ranges, and a low partial dispersion ratio can be obtained.

Further, a third optical glass comprises, in mass %, an SiO₂ component of 10.0 to 70.0%, an Nb₂O₅ component of 1.0 to 50.0%, an Na₂O component of 1.0 to 25.0%, and an Li₂O component of 0.1 to 20.0%, and has a refractive index (nd) of 1.62 to 1.75, an Abbe number (νd) of 30 to 40, and a partial dispersion ratio (θg,F) of no greater than 0.594.

In a glass comprising an SiO₂ component and an Nb₂O₅ component, in particular even in the case of comprising an Na₂O component and an Li₂O component, a glass having a high refractive index and low Abbe number (high dispersion) within the desired ranges, and a low partial dispersion ratio can be obtained.

Accordingly, it is possible to obtain an optical glass useful for reducing chromatic aberrations in optical systems, with a small partial dispersion ratio (θg,F), while having a desired high refractive index (nd) and low Abbe number (νd).

In addition, it is also possible to obtain an optical glass which has a low specific gravity and can therefore contribute to reducing the weight of an optical instrument, which has a high transmittance in the visible range and can be suitably used for applications where visible light is transmitted, and further, which has a low glass transition point and therefore makes it possible to lower the heating temperature when reheat press molding.

Below, embodiments of the optical glass of the present invention are explained in detail, but the present invention is not in any way limited by the below examples, and may be practiced with the addition of suitable modifications within the scope of the objective of the present invention. Further, in portions where the explanations overlap, explanations may be suitably omitted, but this does not limit the gist of the present invention.

[Glass Components]

The compositional ranges of each of the components constituting the optical glass of the present invention are described below. Unless otherwise stated, in the present specification, the content of each component is expressed in mass % with respect to the total mass of the glass of a fully oxide converted composition. Herein, the “oxide converted composition” is a composition where, assuming that the oxides, complex salts, metal fluorides and the like used as raw materials of the constituent components of the glass of the present invention have all been decomposed and converted to oxides when melting the raw materials, each component contained in the glass is stated with the total mass of these oxides as 100 mass %.

<Concerning the Essential Components and Optional Components>

The SiO₂ component is an essential component which stimulates the formation of a stable glass, and reduces devitrification (generation of crystals) which is unfavorable for an optical glass.

In particular, by making the content of the SiO₂ component no less than 10.0%, it is possible to reduce devitrification without greatly increasing the partial dispersion ratio. Further, in this way it is possible to reduce devitrification and coloring when reheating. Accordingly, the content of the SiO₂ component is preferably no less than 10%, more preferably over 20.0%, even more preferably over 25.0%, even more preferably over 30.0%, even more preferably over 32.0%, even more preferably over 34.0%, and even more preferably over 35.0%.

On the other hand, by making the content of the SiO₂ component no greater than 70.0%, reduction of the refractive index is impeded, whereby a desired high refractive index can be readily obtained, and further, increases in the partial dispersion ratio can be restrained. Further, in this way a degradation of the melting properties of the glass raw materials can be restrained. Accordingly, the content of the SiO₂ component is preferably no greater than 70.0%, more preferably less than 60.0%, even more preferably less than 50.0%, even more preferably less than 45.0%, even more preferably less than 43.0%, and even more preferably less than 40.0%.

As a raw material for the SiO₂ component, SiO₂, K₂SiF₆, Na₂SiF₆ and the like may be used.

The Nb₂O₅ component is an essential component and by making its content no less than 1.0%, it is possible to increase the refractive index, and lower the Abbe number and partial dispersion ratio of the glass. Accordingly, the content of the Nb₂O₅ component is preferably no less than 1.0%, more preferably greater than 4.0%, even more preferably greater than 7.0%, even more preferably greater than 10.0%, even more preferably greater than 15.0%, even more preferably greater than 20.0%, even more preferably greater than 23.0%, even more preferably greater than 24.0%, even more preferably greater than 25.0%, and even more preferably greater than 26.0%.

On the other hand, by making the content of the Nb₂O₅ component no greater than 50.0%, it is possible to reduce the material cost of the glass. Further, it is possible to suppress an increase in the melting temperature when manufacturing the glass, and it is further possible to reduce the devitrification due to an excessive content of the Nb₂O₅ component. Accordingly, the content of the Nb₂O₅ component is preferably no greater than 50.0%, more preferably less than 40.0%, even more preferably less than 35.0%, even more preferably less than 31.0%, and even more preferably less than 30.0%.

As a raw material for the Nb₂O₅ component, Nb₂O₅ and the like may be used.

The Na₂O component is an essential component and by making its content no less than 1.0%, it is possible to reduce the partial dispersion ratio of the glass, to enhance the reheat press properties of the glass, the glass transition point can be lowered, and further, the melting properties of the glass raw materials can be enhanced. Accordingly, the content of the Na₂O component is preferably no less than 1.0%, more preferably greater than 3.0%, even more preferably greater than 5.0%, even more preferably greater than 6.0%, even more preferably greater than 8.5%, even more preferably greater than 10.0%, even more preferably greater than 11.0%, and even more preferably greater than 12.0%.

On the other hand, by making the content of the Na₂O component no greater than 30.0%, it is possible to restrain reductions in the refractive index of the glass, deterioration of the chemical resistance of the glass can be impeded, and further, devitrification due to an excessive content can be reduced.

Accordingly, the content of the Na₂O component is preferably no greater than 30.0%, more preferably no greater than 25.0%, even more preferably less than 20.0%, even more preferably less than 18.0%, even more preferably less than 15.0%, and even more preferably less than 13.0%.

As a raw material for the Na₂O component, Na₂CO₃, NaNO₃, NaF, Na₂SiF₆ and the like may be used.

The B₂O₃ component is an optional component, which when having a content exceeding 0%, can promote stable glass formation and reduce devitrification, and further can enhance the melting properties of the glass raw materials. Accordingly, the content of the B₂O₃ component preferably exceeds 0%, more preferably exceeds 1.0%, even more preferably exceeds 3.0%, even more preferably exceeds 4.0%, even more preferably exceeds 5.5%, even more preferably exceeds 7.5%, and even more preferably exceeds 10.0%.

On the other hand, by making the content of the B₂O₃ component no greater than 25.0%, it is possible to restrain reductions in the refractive index and increases in the Abbe number, and it is further possible to restrain increases in the partial dispersion ratio. Accordingly, the content of the B₂O₃ component is preferably no greater than 25.0%, more preferably no greater than 20.0%, even more preferably less than 20.0%, even more preferably less than 16.0%, even more preferably less than 15.0%, and even more preferably less than 12.5%.

As the raw material of the B₂O₃ component, H₃BO₃, Na₂B4O₇, Na₂B4O₇.10H2O, BPO₄ and the like may be used.

The ratio (mass ratio) of the total amount of the Li₂O component and the Na₂O component with respect to the content of the ZrO₂ component is preferably no less than 0.50. In this way, the melting properties of the glass raw materials are enhanced, and it is possible to reduce the devitrification of the glass, and further, the reheat press properties of the glass are enhanced. Accordingly, this (Li₂O+Na₂O)/(ZrO₂) mass ratio preferably has a lower limit of 0.50, more preferably 1.00, even more preferably 1.30, even more preferably 1.70, and even more preferably 1.78.

On the other hand, from the viewpoint of reducing the devitrification of the glass and enhancing the melting properties of the glass raw materials, this (Li₂O+Na₂O)/(ZrO₂) mass ratio is preferably less than 15.00, more preferably less than 12.00, and even more preferably less than 11.00.

The ZrO₂ component is an optional component, which when having a content exceeding 0%, can increase the refractive index of the glass, lower the Abbe number, lower the partial dispersion ratio, and further can reduce devitrification. Further, in this way it is possible to reduce devitrification and coloring when reheating. Accordingly, the content of the ZrO₂ component preferably exceeds 0%, more preferably exceeds 1.0%, even more preferably exceeds 1.5%, even more preferably exceeds 3.0%, even more preferably exceeds 4.0%, even more preferably exceeds 5.0%, and even more preferably exceeds 7.0%.

On the other hand, by making the content of the ZrO₂ component no greater than 25.0%, it is possible to reduce devitrification, and it further becomes easy to obtain a more homogeneous glass. Accordingly, the content of the ZrO₂ component is preferably no greater than 25.0%, more preferably less than 20.0%, even more preferably less than 18.0%, even more preferably less than 16.0%, even more preferably less than 15.0%, even more preferably less than 13.0%, even more preferably less than 10.0%, and even more preferably less than 8.0%.

As the raw material of the ZrO₂ component, ZrO₂, ZrF₄ and the like may be used.

The Li₂O component is an optional component, which when having a content exceeding 0%, can lower the partial dispersion ratio of the glass, enhance the reheat press properties, can lower the glass transition point, and further enhances the melting properties of the glass raw materials. In particular, in the third optical glass, the Li₂O component is an essential component and by making its content no less than 0.1%, it is possible to lower the partial dispersion ratio of the glass, the reheat press properties are enhanced, the glass transition point can be lowered, and further, the melting properties of the glass raw materials are enhanced. Accordingly, the content of the Li₂O component is preferably no less than 0.1%, more preferably greater than 0.5%, even more preferably greater than 1.0%, even more preferably greater than 2.0%, and even more preferably greater than 2.5%.

On the other hand, by making the content of the Li₂O component no greater than 20.0%, it is possible to restrain reductions in the refractive index, deterioration of the chemical resistance can be impeded, and it is further possible to reduce devitrification due to an excessive content.

Accordingly, the content of the Li₂O component is preferably no greater than 20.0%, more preferably less than 10.0%, even more preferably less than 8.0%, even more preferably less than 5.0%, even more preferably less than 3.0%, and even more preferably less than 1.4%.

As the raw material of the Li₂O component, Li₂CO₃, LiNO₃, LiF and the like may be used.

The K2O component is an optional component, which when having a content exceeding 0%, reduces the refractive index, enhances the melting properties of the glass raw materials, and can further lower the glass transition point.

On the other hand, by making the content of the K₂O component no greater than 20.0%, it is possible to restrain increases in the partial dispersion ratio, devitrification can be reduced, and further, deterioration of the chemical resistance can be impeded. Further, it is possible to restrain degradation of the reheat press molding properties. Accordingly, the content of the K₂O component is preferably no greater than 20.0%, more preferably no greater than 15.0%, even more preferably less than 15.0%, even more preferably less than 12.0%, even more preferably less than 11.0%, even more preferably less than 10.0%, even more preferably less than 8.0%, even more preferably less than 5.0%, and even more preferably less than 3.0%.

As the raw material of the K₂O component, K₂CO₃, KNO₃, KF, KHF₂, K₂SiF₆ and the like may be used.

The TiO₂ component is an optional component, which when having a content exceeding 0%, can increase the refractive index, lower the Abbe number, and can further reduce devitrification.

On the other hand, by making the content of the TiO₂ component no greater than 20.0%, it is possible to reduce coloring of the glass, and the internal transmittance is increased. Further, in this way increases in the partial dispersion ratio are impeded, whereby the desired low partial dispersion ratio can be easily obtained. Accordingly, the content of the TiO₂ component is preferably no greater than 20.0%, more preferably less than 15.0%, even more preferably less than 10.0%, even more preferably less than 5.0%, even more preferably less than 3.0%, even more preferably less than 1.0%, and even more preferably less than 0.1%.

As the raw material of the TiO₂ component, TiO₂ and the like may be used.

The MgO component is an optional component, which when having a content exceeding 0%, can lower the melting temperature of the glass.

On the other hand, by making the content of the MgO component no greater than 10.0%, reductions in the refractive index and increases in the Abbe number are suppressed, while devitrification can be reduced. Accordingly, the content of the MgO component is preferably no greater than 10.0%, more preferably less than 5.0%, even more preferably less than 3.0%, and even more preferably less than 1.0%, and even more preferably less than 0.5%.

As the raw material of the MgO component, MgO, MgCO₃, MgF₂ and the like may be used.

The CaO component is an optional component, which when having a content exceeding 0%, reduces the material costs of the glass, while devitrification can be reduced, and further, the melting properties of the glass raw materials are enhanced.

On the other hand, by making the content of the CaO component no greater than 10.0%, reductions in the refractive index and increases in the Abbe number, as well as increases in the partial dispersion ratio are restrained, and further, devitrification can be reduced. Accordingly, the content of the CaO component is preferably no greater than 10.0%, more preferably less than 5.0%, even more preferably less than 3.0%, even more preferably less than 1.9%, and even more preferably less than 0.5%.

As the raw material of the CaO component, CaCO₃, CaF₂ and the like may be used.

The SrO component is an optional component, which when having a content exceeding 0%, can reduce devitrification of the glass, and further increases the refractive index.

In particular, by making the content of the SrO component no greater than 10.0%, increases in the Abbe number are suppressed, and degradation of the chemical resistance is restrained. Accordingly, the content of the SrO component is preferably no greater than 10.0%, more preferably less than 5.0%, even more preferably less than 3.0%, and even more preferably less than 1.0%.

As the raw material of the SrO component, Sr(NO₃)₂, SrF₂ and the like may be used.

The BaO component is an optional component, which when having a content exceeding 0%, can reduce devitrification of the glass, and further, increases the refractive index, enhances the melting properties of the glass raw materials, and further, compared to other alkaline earth components, can reduce the material costs of the glass. Further, it is a component which restrains degradations in the reheat press molding properties.

On the other hand, by making the content of the BaO component no greater than 20.0%, increases in the Abbe number are suppressed, while degradations of the chemical resistance and devitrification are restrained. Accordingly, the content of the BaO component is preferably no greater than 20.0%, more preferably less than 15.0%, even more preferably less than 10.0%, and even more preferably less than 5.0%.

As the raw material of the BaO component, BaCO₃, Ba(NO₃)₂ and the like may be used.

The Ta₂O₅ component is an optional component, which when having a content exceeding 0%, increases the refractive index, lowers the partial dispersion ratio, and further, can reduce devitrification of the glass.

On the other hand, by making the content of the Ta₂O₅ component no greater than 10.0%, it is possible to reduce the used amount of the Ta₂O₅ component which is a rare mineral resource, and further, because the glass readily melts at lower temperatures, the material costs and production costs of the glass can be reduced. Further, in this way it is possible to reduce devitrification and increases in the Abbe number of the glass which are due to an excessive content of the Ta₂O₅ component. Accordingly, the content of the Ta₂O₅ component is preferably no greater than 10.0%, more preferably less than 5.0%, even more preferably less than 3.0%, even more preferably less than 1.0% and even more preferably less than 0.5%. In particular, from the viewpoint of reducing the material costs of the glass, the content of the Ta₂O₅ component may be made less than 0.1%.

As the raw material of the Ta₂O₅ component, Ta₂O₅ and the like may be used.

The La₂O₃ component, Gd₂O₃ component, Y₂O₃ component, and Yb₂O₃ component are optional components, which when at least one of them has a content exceeding 0%, can increase the refractive index, and further can reduce the partial dispersion ratio.

On the other hand, by making the content of the La₂O₃ component no greater than 10.0%, is it possible to restrain increases in the Abbe number, the specific gravity can be made small, and further, devitrification can be reduced. Accordingly, the content of the La₂O₃ component is preferably no greater than 10.0%, more preferably less than 5.0%, even more preferably less than 3.0%, and even more preferably less than 1.0%.

Further, by making the content of the Y₂O₃ component no greater than 20.0%, increases in the Abbe number are restrained, the specific gravity can be made small, and devitrification can be reduced. Accordingly, the content of the Y₂O₃ component is preferably no greater than 20.0%, more preferably less than 10.0%, even more preferably less than 5.0%, and even more preferably less than 3.0%.

Further, by making the content of each of the Gd₂O₃ component and the Yb₂O₃ component no greater than 10.0%, increases in the Abbe number are restrained, the specific gravity can be made small, and devitrification can be reduced, and further material costs can be reduced. Accordingly, the content of each of the Gd₂O₃ component and the Yb₂O₃ component is preferably no greater than 10.0%, more preferably less than 5.0%, even more preferably less than 3.0%, and even more preferably less than 1.0%.

As the raw materials of the La₂O₃ component, Gd₂O₃ component, Y₂O₃ component, and the Yb₂O₃ component, La₂O₃, La(NO₃)3.XH₂O (X is an arbitrary integer), _(Y2O3), YF₃, Gd₂O₃, GdF₃, Yb₂O₃ and the like may be used.

The P₂O₅ component is an optional component, which when having a content exceeding 0%, can reduce devitrification of the glass.

On the other hand, by making the content of the P₂O₅ component no greater than 10.0%, it is possible to reduce devitrification due to an excessive content of the P₂O₅ component. Accordingly, the content of the P₂O₅ component is preferably no greater than 10.0%, more preferably less than 5.0%, even more preferably less than 3.0%, and even more preferably less than 1.0%.

As the raw material of the P₂O₅ component, Al(PO₃)₃, Ca(PO₃)₂, Ba(PO₃)₂, BPO₄, H₃PO₄ and the like may be used.

The GeO₂ component is an optional component, which when having a content exceeding 0%, increases the refractive index, and further can reduce devitrification.

On the other hand, by making the content of the GeO₂ component no greater than 10.0%, because the used amount of expensive GeO₂ is reduced, it is possible to reduce the material costs of the glass. Accordingly, the content of the GeO₂ component is preferably no greater than 10.0%, more preferably less than 5.0%, and even more preferably less than 1.0%.

As the raw material of the GeO₂ component, GeO₂ and the like may be used.

The Al₂O₃ component and Ga₂O₃ component are optional components, which when at least one of them has a content exceeding 0%, increase the chemical resistance, and further can reduce devitrification of the glass.

On the other hand, by making the content of the Al₂O₃ component no greater than 15.0%, it is possible to reduce devitrification due to excessive content. Accordingly, the content of the Al₂O₃ component is preferably no greater than 15.0%, more preferably less than 8.0%, even more preferably less than 5.0%, and even more preferably less than 3.0%.

Further, by making the content of the Ga₂O₃ component no greater than 10.0%, it is possible to reduce devitrification due to excessive content. Accordingly, the content of the Ga₂O₃ component is preferably no greater than 10.0%, more preferably less than 5.0%, and even more preferably less than 3.0%.

As the raw materials of the Al₂O₃ component and Ga₂O₃ component, Al₂O₃, Al(OH)₃, AlF₃, Ga₂O₃, Ga(OH)₃ and the like may be used.

The WO3 component is an optional component, which when having a content exceeding 0%, increases the refractive index and reduces the Abbe number, and can reduce the devitrification of the glass, and further can enhance the melting properties of the raw materials of the glass.

On the other hand, by making the content of the WO₃ component no greater than 10.0%, increases in the partial dispersion ratio of the glass can be impeded, and further, coloring of the glass is reduced and the internal transmittance is increased. Accordingly, the content of the WO₃ component is preferably no greater than 10.0%, more preferably less than 5.0%, even more preferably less than 3.0%, and even more preferably less than 1.0%.

As the raw material of the WO₃ component, WO₃ and the like may be used.

The Bi₂O₃ component is an optional component, which when having a content exceeding 0%, can increase the refractive index and reduce the Abbe number, and further can lower the glass transition point.

On the other hand, by making the content of the Bi₂O₃ component no greater than 10.0%, increases in the partial dispersion ratio can be impeded, and further, coloring of the glass is reduced, and the internal transmittance can be increased. Accordingly, the content of the Bi₂O₃ component is preferably no greater than 10.0%, more preferably less than 5.0%, even more preferably less than 3.0%, and even more preferably less than 1.0%.

As the raw material of the Bi₂O₃ component, Bi₂O₃ and the like may be used.

The ZnO component is an optional component, which when having a content exceeding 0%, can reduce devitrification of the glass, reduces the partial dispersion ratio, and further lowers the glass transition point.

On the other hand, by making the content of the ZnO component no greater than 30.0%, the chemical resistance is increased, while reducing the devitrification and coloring when reheating the glass. Accordingly, the content of the ZnO component is preferably no greater than 30.0%, more preferably less than 20.0%, more preferably less than 10.0%, even more preferably less than 5.0%, even more preferably less than 3.0%, even more preferably less than 2.0%, and even more preferably less than 1.0%.

As the raw material of the ZnO component, ZnO, ZnF₂ and the like may be used.

The TeO₂ component is an optional component, which when having a content exceeding 0%, increases the refractive index, and can lower the partial dispersion ratio, and further can lower the glass transition point.

On the other hand, by making the content of the TeO₂ component no greater than 15.0%, coloring of the glass is reduced and the internal transmittance is increased. Further, by reducing the use of the expensive TeO₂ component, a glass can be obtained at a cheaper material cost. Accordingly, the content of the TeO₂ component is preferably no greater than 15.0%, more preferably less than 10.0%, even more preferably less than 5.0%, even more preferably less than 3.0%, and even more preferably less than 1.0%.

As the raw material of the TeO₂ component, TeO₂ and the like may be used.

The SnO₂ component is an optional component, which when having a content exceeding 0%, can clarify (deaerate) the molten glass, and further, increases the visible light transmittance of the glass.

On the other hand, by making the content of the SnO₂ component no greater than 5.0%, the occurrence of coloring of the glass due to reduction of the molten glass and devitrification of the glass can be impeded. Further, because it is possible to reduce the alloying of the SnO₂ and the melting equipment (in particular precious metals such as Pt and the like), it is possible to plan a lengthening of the service life of the melting equipment. Accordingly, the content of the SnO₂ component is preferably no greater than 5.0%, more preferably less than 3.0%, and even more preferably less than 1.0%.

As the raw material of the SnO2 component, SnO, SnO₂, SnF₂, SnF₄ and the like may be used.

The Sb₂O₃ component is an optional component, which when having a content exceeding 0%, can clarify the glass.

On the other hand, by making the content of the Sb₂O₃ component no greater than 1.0%, the occurrence of excessive foaming is impeded when the glass is molten, whereby alloy formation between the Sb₂O₃ component and the melting equipment (in particular precious metals such as Pt and the like) can be impeded. Accordingly, the content of the Sb₂O₃ component preferably has an upper limit of no greater than 1.0%, more preferably less than 0.5%, and even more preferably less than 0.1%. However, if environmental impact of the optical glass is considered, it is not necessary to include the Sb₂O₃ component.

As the raw material of the Sb₂O₃ component, Sb₂O₃, Sb₂O₅, Na₂H₂Sb₂O₇.5H₂O and the like may be used.

Further, the component clarifying the glass is not limited to the above described Sb₂O₃ component, and well known clarifiers in the field of glass production, or combinations thereof may be used.

The ratio (mass ratio) of the content of the SiO₂ component with respect to the total amount of the SiO₂ component and the B₂O₃ component may be no less than 0.10. In this way, increases in the Abbe number of the glass are restrained. Accordingly, this (SiO₂)/(SiO₂+B₂O₃) mass ratio may preferably have a lower limit of 0.10, more preferably 0.30, even more preferably 0.50, even more preferably 0.65, and even more preferably 0.75.

On the other hand, this (SiO₂)/(SiO₂+B₂O₃) mass ratio preferably may have an upper limit of 1, but from the viewpoints of restraining increases in the glass transition point, reducing devitrification of the glass, and enhancing the melting properties of the glass raw materials, it is preferably less than 1, more preferably less than 0.98, even more preferably no greater than 0.95, even more preferably less than 0.95, even more preferably no greater than 0.93, even more preferably no greater than 0.90, even more preferably no greater than 0.88, even more preferably no greater than 0.83, and even more preferably no greater than 0.80.

The sum (mass sum) of the content of the Rn₂O component (where in the formula, Rn is at least one selected from the group consisting of Li, Na, and K) is preferably from 1.0% to 30.0%.

In particular, by making this mass sum no less than 1.0%, the melting properties of the glass raw materials are enhanced, and further, the glass transition point can be lowered. Therefore, the total content of the Rn₂O component is preferably no less than 1.0%, more preferably greater than 5.0%, even more preferably greater than 10.0%, and even more preferably greater than 12.0%.

On the other hand, by making this mass sum no greater than 30.0%, reductions of the refractive index of the glass can be impeded, and devitrification when molding the glass can be reduced. Accordingly, the total content of the Rn₂O component is preferably no greater than 30.0%, more preferably less than 25.0%, even more preferably less than 23.0%, even more preferably less than 21.0%, even more preferably less than 20.0%, and even more preferably less than 18.0%.

The mass ratio of Li₂O/Rn₂O is preferably no less than 0.01. In this way, it is possible to lower the partial dispersion ratio of the glass, the reheat press properties are enhanced, and the glass transition point can be lowered. Accordingly, this mass ratio of Li₂O/Rn₂O is preferably no less than 0.01, more preferably greater than 0.05, even more preferably greater than 0.10, and even more preferably more than 0.14.

On the other hand, the upper limit of this mass ratio of Li₂O/Rn₂O, from the viewpoint of reducing the devitrification of the glass, is preferably 0.60, more preferably 0.50, and even more preferably 0.40.

The sum (mass sum) of the content of the RO component (where in the formula, R is at least one selected from the group consisting of Mg, Ca, Sr, and Ba) is preferably no greater than 25.0%. In this way, increases in the Abbe number are restrained, and further, it is possible to reduce devitrification of the glass due to an excessive content of these components. Accordingly, the mass sum of the RO component is preferably no greater than 25.0%, more preferably less than 15.0%, even more preferably less than 10.0%, even more preferably less than 5.0%, and even more preferably less than 2.0%.

The sum (mass sum) of the content of the Ln₂O₃ component (where in the formula, Ln is at least one selected from the group consisting of La, Gd, Y, and Yb) is preferably no greater than 20.0%. In this way, it is possible to reduce devitrification of the glass, increases in the Abbe number can be restrained, and further, the material costs can be reduced. Accordingly, the mass sum of the Ln₂O₃ component is preferably no greater than 20.0%, more preferably less than 15.0%, even more preferably less than 10.0%, even more preferably less than 5.0%, even more preferably less than 3.0%, and even more preferably less than 1.0%.

<Concerning Components which should not be Included>

Next, explanations are given for components which should not be included, and components which are unfavorable for inclusion in the optical glass of the present invention.

Other components may be added as necessary within a scope which does not harm the characteristics of the glass of the invention of the present application. However, other than Ti, Zr, Nb, W, La, Gd, Y, Yb, and Lu, all transition metal components such as V, Cr, Mn, Fe, Co, Ni, Cu, Ag, Mo and the like, color the glass even when contained individually or in combinations in small amounts, and because they have the property of giving rise to absorption of specific wavelengths in the visible range, they are preferably substantially not included particularly in an optical glass used for wavelengths in the visible range.

Further, lead compounds such as PbO and the like, and arsenic compounds such as As₂O₃ and the like, are components which have a high environmental load, and therefore, they are substantially not included, namely, it is desired that they are not included at all except for inevitable impurities.

Further, in recent years there has been a tendency to abstain from the use of each of the components of Th, Cd, Tl, Os, Be and Se as harmful chemical materials, and provisions for environmental measures are required not only for the production steps of the glass, but also for the processing steps, and until the disposal after the product has been made. Accordingly, when considering the environmental impact, it is preferable that these are substantially not included.

[Production Method]

The optical glass of the present invention can be produced, for example, as follows. Namely, all of the components of the above described raw materials are uniformly mixed so as to have contents within the prescribed ranges, and the produced mixture is put into a platinum crucible, quartz crucible, or alumina crucible and after rough melting, is put into a gold crucible, platinum crucible, platinum alloy crucible, or iridium crucible, and after carrying out melting for 3 to 5 hrs in a temperature range of 1100 to 1400° C., stirring, homogenizing and defoaming and the like, the temperature is lowered to 1000 to 1400° C., and then the finishing stirring is carried out, stria are removed, and the glass is produced by casting into a mold and annealing.

At this time, it is preferable to use materials having enhanced melting properties as the glass raw materials. In this way, it becomes possible to melt at lower temperatures, and melt for a shorter time, whereby the productivity of the glass is increased, and the production costs can be reduced. Further, because volatilization of the components and reactions with the crucible or the like can be reduced, it is possible to easily obtain a glass with low coloring.

<Physical Properties>

The optical glass of the present invention has a high refractive index and an Abbe number within a prescribed range.

The refractive index (nd) of the optical glass of the present invention preferably has a lower limit of 1.62, more preferably 1.63, even more preferably 1.64, and even more preferably 1.65. The upper limit of this refractive index may preferably be 1.75, more preferably be 1.74, even more preferably 1.72, even more preferably 1.70, and even more preferably 1.68.

The Abbe number (νd) of the optical glass of the present invention is preferably no greater than 40, more preferably no greater than 39, and even more preferably no greater than 38. On the other hand, the Abbe number (νd) of the optical glass of the present invention preferably has a lower limit of 30, more preferably 32, even more preferably 33, and even more preferably 34.

The optical glass of the present invention having such a refractive index and Abbe number is useful for optical design, and in particular even when designing for enhanced imaging characteristics, makes it possible to plan a size reduction of the optical system, and therefore, allows a wide freedom of optical design.

Herein, the refractive index (nd) and the Abbe number (νd) of the optical glass of the present invention preferably satisfy the relationship (−0.012νd+2.04)≤nd≤(−0.012νd+2.16). In the glass composition specified by the present invention, when the refractive index (nd) and the Abbe number (νd) satisfy this relationship, a glass where the occurrence of devitrification is further impeded can be obtained.

Accordingly, in the optical glass of the present invention, the refractive index (nd) and the Abbe number (νd) preferably satisfy the relationship nd≥(−0.012νd+2.04), more preferably satisfy the relationship nd≥(−0.012νd+2.05), even more preferably satisfy the relationship nd≥(−0.012νd+2.06), and even more preferably satisfy the relationship nd≥(−0.012νd+2.08).

On the other hand, in the optical glass of the present invention, the refractive index (nd) and the Abbe number (νd) preferably satisfy the relationship nd≤(−0.012νd+2.16), more preferably satisfy the relationship nd≤(−0.012νd+2.14), even more preferably satisfy the relationship nd≤(−0.012νd+2.13), and even more preferably satisfy the relationship nd≤(−0.012νd+2.12).

The optical glass of the present invention has a low partial dispersion ratio (θg,F).

More specifically, the partial dispersion ratio (θg,F) of the optical glass of the present invention preferably has an upper limit of 0.594, more preferably 0.592, even more preferably 0.590, and even more preferably 0.588. The lower limit of this partial dispersion ratio (θg,F) is preferably 0.570, and may be more preferably 0.573, even more preferably 0.575, even more preferably 0.576, and even more preferably 0.577.

Further, the partial dispersion ratio (θg,F) of the optical glass of the present invention and the Abbe number (νd) preferably satisfy the relationship (−0.00162×νd+0.630)≤(θg,F)≤(−0.00162×νd+0.652).

In this way, because it is possible to obtain an optical glass having a low partial dispersion ratio (θg,F), an optical element formed from this optical glass is useful for reducing chromatic aberrations of an optical system.

Accordingly, in the optical glass of the present invention, the partial dispersion ratio (θg,F) and the Abbe number (νd) preferably satisfy the relationship θg,F≥(−0.00162×νd+0.630), more preferably satisfy the relationship θg,F≥(−0.00162×νd+0.632), and even more preferably satisfy the relationship θg,F≥(−0.00162×νd+0.634).

On the other hand, in the optical glass of the present invention, the partial dispersion ratio (θg,F) and the Abbe number (νd) preferably satisfy the relationship θg,F≤(−0.00162×νd+0.652), more preferably satisfy the relationship θg,F≤(−0.00162×νd+0.650), even more preferably satisfy the relationship θg,F≤(−0.00162×νd+0.648), even more preferably satisfy the relationship θg,F≤(−0.00162×νd+0.646), and even more preferably satisfy the relationship θg,F≤(−0.00162×νd+0.643).

Further, with the above relational formulas of the partial dispersion ratio (θg,F) and the Abbe number (νd), by stipulating these relationships using a straight line with the same inclination as the normal line, it is shown that a glass having a smaller partial dispersion ratio (θg,F) than a typical glass can be obtained.

The optical glass of the present invention preferably has a small specific gravity. More specifically, the specific gravity of the optical glass of the present invention is preferably no greater than 3.80 [g/cm3]. In this way, because the weight of the optical element and an optical device using the same can be reduced, this can contribute to optical devices with reduced weight. Accordingly, the specific gravity of the optical glass of the present invention preferably has an upper limit of 3.80, more preferably has an upper limit of 3.50, even more preferably 3.30, even more preferably 3.10, and even more preferably 3.00. Further, the specific gravity of the optical glass of the present invention is often generally no less than 2.50, more specifically no less than 2.70, and even more specifically no less than 2.80.

The specific gravity of the optical glass of the present invention is measured based on the Japan Optical Glass Industrial Standard JOGIS 05-1975 “Measuring Method of Specific Gravity of Optical Glass”.

The optical glass of the present invention preferably has low coloring.

In particular, for the optical glass of the present invention, the wavelength (Δ5) showing a spectral transmittance of 5% for a sample with a thickness of 10 mm is preferably no greater than 400 nm, more preferably no greater than 380 nm, and even more preferably no greater than 350 nm.

Further, for the optical glass of the present invention, the wavelength showing a spectral transmittance (λ80) of 80% for a sample with a thickness of 10 mm is preferably no greater than 450 nm, more preferably no greater than 420 nm, and even more preferably 400 nm.

In this way, the absorption edge of the glass becomes located in the vicinity of the ultraviolet region, and the transparency of the glass in the visible region is increased, whereby this optical glass can be favorably used as a material of optical elements of lenses and the like.

The optical glass of the present invention preferably has a glass transition point of no greater than 650° C. In this way, because the glass softens at a lower temperature, it is possible to mold press mold the glass at a lower temperature. Further, the oxidation of the die used for the mold press molding is reduced and a long design life can be planned. Accordingly, the glass transition point of the optical glass of the present invention preferably has an upper limit of 650° C., more preferably 620° C., even more preferably 600° C., even more preferably 580° C., and even more preferably 550° C.

Further, the lower limit of the glass transition point of the optical glass of the present invention is not particularly limited, but the glass transition point of the optical glass of the present invention preferably has a lower limit of 460° C., more preferably 480° C., and even more preferably 500° C.

The optical glass of the present invention preferably has a yield point (At) of no greater than 720° C. The yield point, similarly to the glass transition point, is one indicator showing the softening properties of the glass, and is an indicator which shows a temperature close to the press molding temperature. Therefore, by using a glass with a yield point of no greater than 720° C., because it is possible to press mold at a lower temperature, it is possible to more easily carry out press molding. Accordingly, the yield point of the optical glass of the present invention preferably has an upper limit of 720° C., more preferably 700° C., even more preferably 690° C., even more preferably 680° C., even more preferably 660° C., even more preferably 650° C., and even more preferably 630° C.

Further, the lower limit of the yield point of the optical glass of the present invention is not particularly limited, but the lower limit is preferably 500° C., more preferably 530° C., and even more preferably 560° C.

The optical glass of the present invention preferably has a small average coefficient of linear expansion (α). In particular, the upper limit of the average coefficient of linear expansion of the optical glass of the present invention is preferably 150×10-7 K-1, more preferably 120×10-7 K-1, even more preferably 115×10-7 K-1, even more preferably 110×10-7 K-1, and even preferably 100×10-7 K-1. In this way, when press molding an optical glass with a molding die, the total amount of expansion and contraction due to temperature changes of the glass are reduced. Therefore, breakage of the optical glass when press molding can be impeded, and the productivity of the optical element can be increased.

The optical glass of the present invention is preferably one which has suitable reheat press molding properties. More specifically, in the optical glass of the present invention, devitrification and opacity preferably do not occur before or after reheating tests (drop tests). In this way, by impeding the occurrence of devitrification and coloring even in reheating tests which assume reheat press processing, because the loss of light transmittance of the glass is impeded, it become easy to carry out reheating processes represented by reheat press processing for the glass. Namely, because it is possible to manufacture optical elements of complex shapes by press molding, the production costs can be made inexpensive, and further, it is possible to implement optical element production with good productivity.

Herein, the reheating test (drop test) can be carried out by a method of mounting a 15 mm×15 mm×30 mm test piece on a concave refractory body and inserting it into an electric furnace and reheating, increasing the temperature from normal temperature over 150 min to a temperature which is 80° C. to 150° C. higher than the transition temperature (Tg) of each test piece (the temperature at which it collapses into the refractory body), and after holding at this temperature for 30 min, cooling to normal temperature and removing it from the furnace, polishing two opposing faces to a thickness of 10 mm so that the inner portion can be observed, after which the polished test piece is visually observed.

Further, the presence or absence of devitrification and opacity before and after the reheating test (drop test) can be confirmed for example by visual inspection, and “no occurrence of devitrification and opacity” indicates for example that the transmittance of a light beam (d line) with a wavelength of 587.56 nm for a test piece after the reheating test (drop test) divided by the transmittance of the d line of the test piece before the reheating test, has a value of roughly no less than 0.80.

The optical glass of the present invention preferably has a high chemical resistance. More specifically, the optical glass of the present invention preferably has a high water resistance and acid resistance. In this way, when carrying out the polishing process of the optical glass, because clouding of the glass due to a washing fluid and polishing fluid is reduced, the polishing process can be more easily carried out.

Further, the water resistance and acid resistance of the optical glass indicates that the chemical resistance (water resistance and acid resistance) is preferably grade 1 to 3, more preferably grade 1 to 2, and even more preferably grade 1 according to Japanese Optical Glass Industrial Standard “Method of Measuring Chemical Resistance of Optical Glass “JOGIS 06-2008.

The optical glass of the present invention is preferably one where the occurrence of devitrification when producing the glass is impeded. In this way, because reductions in the transmissivity due to crystallization and the like of the glass are restrained when producing the glass, it is possible to favorably use this optical glass for optical elements which transmit visible light such as lenses and the like. Further, as a gauge showing the degree of impeding the occurrence of devitrification when producing the glass, for example, a low liquid phase temperature can be mentioned.

[Preform and Optical Element]

A glass molded body can be produced from the produced optical glass, for example, using a mold press molding means such as reheat press molding or a precision press molding or the like. Namely, a preform for mold press molding is produced from the optical glass, and a glass molded body can be produced by carrying out reheat press molding of this preform and then carrying out a polishing process, or for example, a glass molded body can be produced by carrying out precision press molding on a preform produced by carrying out a polishing process. Further, the means for producing the glass molded body are not limited to these means.

A glass molded body produced in this way is useful for various optical elements, but among them, it is particularly suitable for use in applications of optical elements such as lenses or prisms, and the like. In this way, blurring of colors due to chromatic aberrations in the transmitted light of an optical system provided with the optical elements is reduced. Therefore, when these optical elements are used for a camera, the photographed objects can be more correctly represented, and when these optical elements are used for a projector, the desired image can be more brilliantly projected.

EXAMPLES

The compositions of Examples (No. A1 to No. A27, No. B1 to No. B11, and No. C1 to No. C12) of the present invention, and the results for the refractive index (nd), the Abbe number (νd), partial dispersion ratio (θg,F), wavelengths (λ5, λ80) showing spectral transmittances of 5% and 80%, glass transition point (Tg), yield point (At), average coefficient of linear expansion (α), and specific gravity, are shown in Tables 1 to 8. Herein, Examples (No. A1 to No. A27) may be taken as examples of first the optical glass; Examples (No. B1 to No. B11) may be taken as examples of the second optical glass, and Examples (No. C1 to No. C12) may be taken as examples of the third optical glass, but they are not limited to this. Further, the below examples are merely for the purpose of illustration, and the present invention is not in any way limited to these examples.

For the glasses of the examples, high purity raw materials usually used for optical glass such as all of the oxides, hydroxides, carbonates, nitrates, fluorides, hydroxides, metaphosphoric acid compounds and the like respectively corresponding to each of the component raw materials were selected, weighed so as to attain the proportions of the compositions of each of the Examples and Comparative Examples shown in the tables and uniformly mixed, were then introduced into a platinum crucible, and depending on the degree of difficulty or ease of melting the glass raw materials, were melted in an electric furnace for 3 to 5 hours in a temperature range of 1100 to 1400° C., stirring, homogenizing and defoaming and the like were carried out, after which the temperature was reduced to 1000 to 1400° C., stirring and homogenizing were carried out, followed by casting into a mold, then annealing, and the glass was produced.

The refractive index (nd), Abbe number (νd), and partial dispersion ratio (θg,F) of the glasses of the examples were measured based on Japanese Optical Glass Industrial Standard JOGIS 01-2003.

Then, from the obtained values of the refractive index (nd) and Abbe number (νd), in the relation (nd=−a×νd+b), the intercept b for a slope a of 0.012 was obtained.

Further, from the obtained values of the Abbe number (νd) and the partial dispersion ratio (θg,F), in the relation (θg,F=−a′ λ νd+b′), the intercept b′ for a slope a′ of 0.00162 was obtained.

Further, the glasses used in the present measurements are ones for which the treatment was carried out in an annealing furnace at an annealing temperature reduction rate of −25° C./hr.

The transmittance of the glasses of the examples was measured according to Japanese Optical Glass Industrial Standard JOGIS 02. Further, in the present invention, by measuring the transmittance of the glass, the presence or absence of coloring of the glass and the degree thereof were determined. Specifically, the spectral transmittance from 200 to 800 nm was measured based on JISZ 8722 for an opposing surface parallel polished product with a thickness of 10±0.1 mm, and A5 (wavelength when the transmittance is 5%) and λ80 (wavelength when the transmittance is 80%) were determined.

The glass transition point (Tg) and yield point (At) of the glasses of the examples were determined by the thermal expansion curve obtained by measuring the relationship between the temperature and the expansion of a test piece according to Japanese Optical Glass Industrial Standard JOGIS 08-2003, “Method of Measuring Thermal Expansion of an Optical Glass”.

The average coefficient of linear expansion (a) of the glasses of the examples was determined by the average linear expansion coefficient at 100 to 300° C. according to Japanese Optical Glass Industrial Standard JOGIS 08-2003, “Method of Measuring Thermal Expansion of an Optical Glass”.

The specific gravity of the glasses of the examples was measured based on Japanese Optical Glass Industrial Standard JOGIS 05-1975 “Measuring Method of Specific Gravity of Optical Glass”.

TABLE 1 Examples (Units: mass %) A1 A2 A3 A4 A5 A6 A7 A8 SiO₂ 37.72 42.23 35.14 40.10 36.27 39.68 39.38 38.67 Nb₂O₅ 26.28 25.69 28.64 27.94 29.56 27.65 27.43 26.95 Na₂O 12.70 12.41 6.91 6.74 14.29 13.36 14.73 13.02 B₂O₃ 10.50 7.16 7.99 4.41 11.81 11.05 10.96 10.77 ZrO₂ 12.70 12.41 7.72 7.53 7.97 7.45 7.39 10.14 Li₂O 2.97 2.90 0.72 0.35 K₂O 10.54 10.28 ZnO Sb₂O₃ 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 (Li + Na)/Zr 1.00 1.00 1.28 1.28 1.79 1.89 1.99 1.32 Si/(Si + B) 0.78 0.85 0.81 0.90 0.75 0.78 0.78 0.78 Li + Na + K 12.70 12.41 20.42 19.92 14.29 14.08 14.73 13.37 Li/(Li + Na + K) 0.00 0.00 0.15 0.15 0.00 0.05 0.00 0.03 Mg + Ca + Sr + Ba 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 La + Gd + Y + Yb 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Refractive index (n_(d)) 1.669 1.663 1.673 1.665 1.671 1.660 1.656 1.664 Abbe number (ν_(d)) 35.3 35.9 36.6 37.3 34.7 36.3 36.3 35.8 Intercept b (a = 0.012) 2.09 2.09 2.11 2.11 2.09 2.09 2.09 2.09 Partial dispersion 0.582 0.580 0.579 0.576 0.589 0.581 0.580 0.581 ratio (θg, F) Intercept b′ (a′ = 0.00162) 0.639 0.638 0.638 0.637 0.645 0.639 0.639 0.639 λ₈₀ [nm] 376 371 367 374 382 375 372 378 λ₅ [nm] 335 334 327 325 335 333 332 334 Specific gravity 3.00 2.99 3.02 3.00 2.99 2.96 2.96 2.98 Glass transition 588 614 520 542 576 561 563 574 point Tg [° C.] Yield point 657 685 584 612 641 628 625 640 At [° C.] Average coefficient 77 78 101 102 83 85 89 81 of linear expansion (α) [×10⁻⁷ K⁻¹]

TABLE 2 Examples (Units: mass %) A9 A10 A11 A12 A13 A14 A15 A16 SiO₂ 40.27 39.02 35.42 36.26 26.16 39.47 37.37 37.73 Nb₂O₅ 25.92 26.06 28.78 29.46 36.94 27.75 26.04 26.29 Na₂O 12.99 15.19 20.13 26.72 11.62 7.61 12.58 12.71 B₂O₃ 10.18 10.24 7.14 2.57 11.61 11.15 10.40 10.50 ZrO₂ 10.17 9.01 8.08 4.55 11.59 11.14 9.79 9.89 Li₂O 0.36 0.37 0.36 0.37 1.95 2.78 0.34 0.34 K₂O P₂O₅ 3.37 Al₂O₃ 2.44 Sb₂O₃ 0.11 0.11 0.11 0.07 0.13 0.11 0.10 0.10 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 (Li + Na)/Zr 1.31 1.73 2.54 5.95 1.17 0.93 1.32 1.32 Si/(Si + B) 0.80 0.79 0.83 0.93 0.69 0.78 0.78 0.78 Li + Na + K 13.36 15.56 20.49 27.08 13.57 10.39 12.92 13.05 Li/(Li + Na + K) 0.03 0.02 0.02 0.01 0.14 0.27 0.03 0.03 Mg + Ca + Sr + Ba 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 La + Gd + Y + Yb 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Refractive index (n_(d)) 1.664 1.658 1.671 1.653 1.742 1.677 1.656 1.657 Abbe number (ν_(d)) 36.0 36.9 36.1 36.6 30.6 35.4 36.2 35.9 Intercept b (a = 0.012) 2.10 2.10 2.10 2.09 2.11 2.10 2.09 2.09 Partial dispersion 0.583 0.581 0.581 0.583 0.592 0.588 0.592 0.579 ratio (θg, F) Intercept b′ (a′ = 0.00162) 0.641 0.641 0.640 0.642 0.642 0.645 0.651 0.637 λ₈₀ [nm] 391 374 378 370 412 387 375 383 λ₅ [nm] 339 330 327 322 343 338 335 337 Specific gravity 3.22 2.99 3.07 3.04 3.22 2.98 2.95 2.95 Glass transition 621 581 578 536 535 540 619 568 point Tg [° C.] Yield point 699 647 641 596 589 600 635 616 At [° C.] Average coefficient 77 88 108 134 91 73 79 80 of linear expansion (α) [×10⁻⁷ K⁻¹]

TABLE 3 Examples (Units: mass %) A17 A18 A19 A20 A21 A22 A23 A24 SiO₂ 35.81 34.88 37.91 38.29 38.15 37.71 37.27 38.02 Nb₂O₅ 24.95 24.31 26.42 26.69 26.59 26.28 25.97 26.50 Na₂O 12.06 11.75 12.77 12.90 12.85 12.70 12.55 12.80 B₂O₃ 9.97 9.71 10.55 10.66 10.62 10.50 10.37 12.27 ZrO₂ 9.38 9.14 9.93 10.03 10.00 9.88 9.77 9.96 Li₂O 0.32 0.31 0.34 0.35 0.34 0.34 0.34 0.34 MgO 0.98 CaO 1.36 SrO 2.48 BaO 3.63 Ta₂O₅ 9.80 La₂O₃ 7.41 Al₂O₃ ZnO 1.96 Sb₂O₃ 0.10 0.09 0.10 0.10 0.10 0.10 0.10 0.10 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 (Li + Na)/Zr 1.32 1.32 1.32 1.32 1.32 1.32 1.32 1.32 Si/(Si + B) 0.78 0.78 0.78 0.78 0.78 0.78 0.78 0.76 Li + Na + K 12.38 12.06 13.11 13.24 13.19 13.04 12.89 13.15 Li/(Li + Na + K) 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 Mg + Ca + Sr + Ba 0.00 0.00 0.00 0.98 1.36 2.48 3.63 0.00 La + Gd + Y + Yb 7.41 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Refractive index (n_(d)) 1.686 1.692 1.667 1.665 1.667 1.675 1.676 1.670 Abbe number (ν_(d)) 36.2 33.7 35.7 36.1 36.2 36.0 35.8 35.9 Intercept b (a = 0.012) 2.12 2.10 2.10 2.10 2.10 2.11 2.11 2.10 Partial dispersion 0.586 0.586 0.579 0.581 0.580 0.586 0.579 0.587 ratio (θg, F) Intercept b′ (a′ = 0.00162) 0.645 0.641 0.637 0.640 0.638 0.644 0.637 0.645 λ₈₀ [nm] 439 393 380 379 381 383 382 381 λ₅ [nm] 342 342 334 333 333 337 337 335 Specific gravity 3.18 3.21 3.02 2.99 3.00 3.07 3.10 3.03 Glass transition 586 586 561 572 578 579 581 575 point Tg [° C.] Yield point 650 649 629 637 637 643 648 612 At [° C.] Average coefficient 79 70 80 78 82 75 71 79 of linear expansion (α) [×10⁻⁷ K⁻¹]

TABLE 4 Examples (Units: mass %) A25 A26 A27 SiO₂ 37.80 38.61 38.60 Nb₂O₅ 26.34 26.40 28.00 Na₂O 12.73 13.51 14.00 B₂O₃ 10.52 11.31 10.80 ZrO₂ 9.90 9.92 8.00 Li₂O 0.34 0.40 K₂O 2.26 Sb₂O₃ 0.10 0.25 0.20 Total 100.0 100.0 100.0 (Li + Na)/Zr 1.32 1.36 1.80 Si/(Si + B) 0.78 0.77 0.78 Li + Na + K 15.33 13.51 14.40 Li/(Li + Na + K) 0.02 0.00 0.03 Mg + Ca + Sr + Ba 0.00 0.00 0.00 La + Gd + Y + Yb 0.00 0.00 0.00 Refractive index (n_(d)) 1.668 1.659 1.640 Abbe number (ν_(d)) 36.0 36.0 40.3 Intercept b (a = 0.012) 2.10 2.09 2.12 Partial dispersion 0.582 0.582 0.571 ratio (θg, F) Intercept b′ (a′ = 0.00162) 0.641 0.640 0.636 λ₈₀ [nm] 378 386 357 λ₅ [nm] 335 336 320 Specific gravity 3.02 2.97 3.13 Glass transition 569 583 595 point Tg [° C.] Yield point 613 645 664 At [° C.] Average coefficient 81 79 84 of linear expansion (α) [×10⁻⁷ K⁻¹]

TABLE 5 Examples (Units: mass %) B1 B2 B3 B4 B5 B6 B7 B8 SiO₂ 34.76 34.49 34.22 35.02 36.27 36.72 36.11 39.68 Nb₂O₅ 28.33 28.11 27.89 28.54 29.56 29.93 29.44 27.65 Na₂O 13.69 15.10 13.48 13.79 14.29 14.46 17.39 13.36 Li₂O 0.73 1.55 0.72 ZrO₂ 7.63 7.57 7.51 7.69 7.97 1.66 1.64 7.45 B₂O₃ 14.75 14.63 14.52 14.86 11.81 15.58 15.32 11.05 K₂O 2.28 Sb₂O₃ 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 (Li + Na)/Zr 1.89 1.99 1.79 1.79 1.79 9.63 10.63 1.89 Si/(Si + B) 0.70 0.70 0.70 0.70 0.75 0.70 0.70 0.78 Li + Na + K 14.43 15.1 15.76 13.79 14.29 16.02 17.39 14.08 Li/(Li + Na + K) 0.05 0.00 0.00 0.00 0.00 0.10 0.00 0.05 Mg + Ca + Sr + Ba 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 La + Gd + Y + Yb 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Refractive index (n_(d)) 1.665 1.662 1.660 1.662 1.671 1.653 1.646 1.660 Abbe number (ν_(d)) 35.6 35.6 35.9 35.3 34.7 37.2 37.4 36.3 Intercept b (a = 0.012) 2.09 2.09 2.09 2.09 2.09 2.10 2.09 2.09 Partial dispersion 0.581 0.581 0.584 0.586 0.589 0.579 0.579 0.581 ratio (θg, F) Intercept b′ (a′ = 0.00162) 0.639 0.639 0.642 0.643 0.645 0.639 0.639 0.639 λ₈₀ [nm] 372 370 370 375 382 371 366 375 λ₅ [nm] 333 333 333 336 335 330 329 333 Specific gravity 2.96 2.96 2.96 2.95 2.99 2.92 2.92 2.96 Glass transition 544 563 556 558 576 530 561 561 point Tg [° C.] Yield point 605 606 621 623 641 588 621 628 At [° C.] Average coefficient 88 89 89 83 83 95 96 85 of linear expansion (α) [×10⁻⁷ K⁻¹]

TABLE 6 Examples (Units: mass %) B9 B10 B11 SiO₂ 39.38 35.42 38.6 Nb₂O₅ 27.43 28.78 28 Na₂O 14.73 20.13 14.00 Li₂O 0.00 0.36 0.40 ZrO₂ 7.39 8.08 8.00 B₂O₃ 10.96 7.14 10.80 K₂O Sb₂O₃ 0.10 0.11 0.20 Total 100.0 100.0 100.0 (Li + Na)/Zr 1.99 2.54 1.8 Si/(Si + B) 0.78 0.83 0.78 Li + Na + K 14.73 20.49 14.4 Li/(Li + Na + K) 0.00 0.02 0.03 Mg + Ca + Sr + Ba 0.00 0.00 0.00 La + Gd + Y + Yb 0.00 0.00 0.00 Refractive index (n_(d)) 1.656 1.671 1.640 Abbe number (ν_(d)) 36.3 36.1 40.3 Intercept b (a = 0.012) 2.09 2.10 2.12 Partial dispersion 0.58 0.581 0.571 ratio (θg, F) Intercept b′ (a′ = 0.00162) 0.639 0.64 0.636 λ₈₀ [nm] 372 378 357 λ₅ [nm] 332 327 320 Specific gravity 2.96 3.07 3.13 Glass transition 563 578 595 point Tg [° C.] Yield point 625 641 664 At [° C.] Average coefficient 89 108 84 of linear expansion (α) [×10⁻⁷ K⁻¹]

TABLE 7 Examples (Units: mass %) C1 C2 C3 C4 C5 C6 SiO₂ 36.32 35.14 40.10 31.99 30.27 31.26 Nb₂O₅ 29.60 28.64 27.94 26.08 30.37 25.48 Na₂O 7.14 6.91 6.74 12.60 11.93 12.31 Li₂O 3.45 2.97 2.90 3.04 2.56 2.64 ZrO₂ 7.98 7.72 7.53 12.60 11.93 15.04 B₂O₃ 15.41 7.99 4.41 13.57 12.84 13.26 K₂O 10.54 10.28 Sb₂O₃ 0.10 0.10 0.10 0.10 0.10 Total 100.0 100.0 100.0 100.0 100.0 100.0 (Li + Na)/Zr 1.33 1.28 1.28 1.24 1.21 0.99 Si/(Si + B) 0.70 0.81 0.90 0.70 0.70 0.70 Li + Na + K 10.60 20.42 19.92 15.65 14.49 14.96 Li/(Li + Na + K) 0.33 0.15 0.15 0.19 0.18 0.18 Mg + Ca + Sr + Ba 0.00 0.00 0.00 0.00 0.00 0.00 La + Gd + Y + Yb 0.00 0.00 0.00 0.00 0.00 0.00 Refractive index (n_(d)) 1.676 1.673 1.665 1.683 1.703 1.688 Abbe number (ν_(d)) 35.5 36.6 37.3 36.3 33.9 35.9 Intercept b (a = 0.012) 2.10 2.11 2.11 2.12 2.11 2.12 Partial dispersion 0.587 0.579 0.576 0.576 0.586 0.581 ratio (θg, F) Intercept b′ (a′ = 0.00162) 0.645 0.638 0.637 0.635 0.641 0.639 λ₈₀ [nm] 381 367 374 371 380 373 λ₅ [nm] 339 327 325 329 335 330 Specific gravity 2.94 3.02 3.00 3.04 3.10 3.06 Glass transition 515 520 542 523 531 point Tg [° C.] Yield point 573 584 612 580 591 At [° C.] Average coefficient 79 101 102 99 97 of linear expansion (α) [×10⁻⁷ K⁻¹]

TABLE 8 Examples (Units: mass %) C7 C8 C9 C10 C11 C12 SiO₂ 33.00 26.16 39.47 19.06 19.06 19.00 Nb₂O₅ 26.90 36.94 27.75 39.00 37.00 36.00 Na₂O 10.00 11.62 7.61 7.94 7.94 7.94 Li₂O 3.00 1.95 2.78 4.74 4.74 4.74 ZrO₂ 13.00 11.59 11.14 13.13 13.13 12.50 B₂O₃ 14.00 11.61 11.15 16.00 18.00 19.70 Sb₂O₃ 0.10 0.13 0.11 0.12 0.12 0.12 Total 100.0 100.0 100.0 100.0 100.0 100.0 (Li + Na)/Zr 1.00 1.17 0.93 0.97 0.97 1.01 Si/(Si + B) 0.70 0.69 0.78 0.54 0.51 0.49 Li + Na + K 13.00 13.57 10.39 12.68 12.68 12.68 Li/(Li + Na + K) 0.23 0.14 0.27 0.37 0.37 0.37 Mg + Ca + Sr + Ba 0.00 0.00 0.00 0.00 0.00 0.00 La + Gd + Y + Yb 0.00 0.00 0.00 0.00 0.00 0.00 Refractive index (n_(d)) 1.685 1.742 1.677 1.720 1.720 1.692 Abbe number (ν_(d)) 35.7 30.6 35.4 36.1 35.5 38.9 Intercept b (a = 0.012) 2.11 2.11 2.10 2.15 2.15 2.16 Partial dispersion 0.581 0.592 0.588 0.576 0.581 0.570 ratio (θg, F) Intercept b′ (a′ = 0.00162) 0.638 0.642 0.645 0.635 0.639 0.633 λ₈₀ [nm] 412 387 379 390 357 λ₅ [nm] 343 338 319 326 309 Specific gravity 3.03 3.22 2.98 3.58 3.44 3.50 Glass transition 526 535 540 518 513 514 point Tg [° C.] Yield point 584 589 600 574 564 569 At [° C.] Average coefficient 88 91 73 91 95 89 of linear expansion (α) [×10⁻⁷ K⁻¹]

According to these tables, the optical glasses of the examples had a partial dispersion ratio (θg,F) of no greater than 0.594, and more specifically no greater than 0.593, which was within the desired range. In particular, the optical glasses of Examples (No. B1 to No. B11) had a partial dispersion ratio (θg,F) of no greater than 0.590.

Herein, for the optical glass of the examples of the present invention, the partial dispersion ratio (θg,F) and the Abbe number (νd) satisfied the relationship (−0.00162×νd+0.630)≤(θg,F)≤(−0.00162×νd+0.652). In particular, the optical glasses of Examples (No. C1 to No. C12) satisfied the relationship (θg,F)≤(−0.00162×νd+0.650). Further, the optical glasses of Examples (No. B1 to No. B11) satisfied the relationship (θg,F)≤(−0.00162×νd+0.647). Moreover, the relationship between the partial dispersion ratio (θg,F) and the Abbe number (νd) for the optical glasses of the examples of the present specification was as shown in FIG. 2.

Accordingly, it was clear that the optical glasses of the examples of the present invention had a small partial dispersion ratio (θg,F).

The optical glasses of the examples of the present invention all had a refractive index (nd) of no less than 1.62, and more specifically no less than 1.64, and were within the desired range. In particular, the optical glasses of Examples (No. C1 to C12) had a refractive index (nd) of no less than 1.66.

Further, the refractive index (nd) of the optical glasses of the present invention were no greater than 1.75. In particular, the optical glasses of Examples (No. B1 to B11) had a refractive index (nd) of no greater than 1.68.

Further, the optical glasses of the examples of the present invention all had an Abbe number (νd) of no less than 30, and besides, these Abbe numbers (νd) were no greater than 42, more specifically no greater than 41, and were within the desired range. In particular, the optical glasses of Examples (No. A1 to No. A27, and No. B1 to No. B11) had an Abbe number of no less than 34. On the other hand, the optical glasses of Examples (No. C1 to No. C12) had an Abbe number of no more than 39.

Herein, for the optical glasses of the examples of the present invention, the refractive index (nd) and the Abbe number (νd) satisfied the relationship (−0.012νd+2.04)≤nd≤(−0.012νd+2.16), and more specifically satisfied the relationship (−0.012νd+2.08)≤nd≤(−0.012νd+2.16). In particular, for the optical glasses of Examples (No. A1 to A27) the refractive index (nd) and the Abbe number (νd) satisfied the relationship (−0.012νd+2.08)≤nd≤(−0.012νd+2.13). Further, for the optical glasses of Examples (No. B1 to B11) the refractive index (nd) and the Abbe number (νd) satisfied the relationship (−0.012νd+2.08)≤nd≤(−0.012νd+2.11). Further, for the optical glasses of Examples (No. C1 to C12) the refractive index (nd) and the Abbe number (νd) satisfied the relationship (−0.012νd+2.09)≤nd≤(−0.012νd+2.16). Moreover, the relationship between the refractive index (nd) and the Abbe number (νd) for the glasses of the examples of the present application were as shown in FIG. 3.

Accordingly, it was clear that the optical glasses of the examples are optical glasses which have a refractive index (nd) and an Abbe number (νd) within the desired ranges, and further, which have a low partial dispersion ratio (θg,F).

In addition, the optical glasses of the Examples all had a λ5 (wavelength when the transmittance is 5%) of no greater than 400 nm, and more specifically no greater than 350 nm. In particular, the optical glasses of Examples (No. B1 to B11) had a λ5 (wavelength when the transmittance is 5%) of no greater than 340 nm.

Further, the optical glasses of the Examples all had a Δ80 (wavelength when the transmittance is 80%) of no greater than 450 nm, more specifically no greater than 440 nm. In particular, the optical glasses of Examples (No. B1 to B11) had a Δ5 (wavelength when the transmittance is 5%) of no greater than 390 nm. Further, the optical glasses of Examples (No. C1 to C12) had a Δ80 (wavelength when the transmittance is 80%) of no greater than 420 nm.

Accordingly, it was clear that the optical glasses of the examples had a high transmittance for visible light, and low coloring.

In addition, the optical glasses of the examples all had a specific gravity of no greater than 3.80, more specifically no greater than 3.60, and were within the desired range. In particular, the optical glasses of Examples (No. A1 to A27) had a specific gravity of no greater than 3.30. Further, the optical glasses of Examples (No. B1 to B11) had a specific gravity of no greater than 3.00.

Further, for the optical glasses of the Examples, the glass transition point was no greater than 650° C., more specifically no greater than 630° C. In particular, the optical glasses of Examples (No. B1 to No. B11) had a glass transition point of no greater than 600° C. Further, the optical glasses of Examples (No. C1 to No. C12) had a glass transition point of no greater than 550° C.

Further, the optical glasses of the examples all had a yield point of no greater than 700° C., and were within the desired range. In particular, the optical glasses of Examples (No. B1 to No. B11) had a yield point of no greater than 670° C. Further, the optical glasses of Examples (No. C1 to No. C12) had a yield point of no greater than 620° C.

In this way, it can be surmised that it is possible to mold press mold the glass at a lower temperature.

Further, the optical glasses of the examples had an average coefficient of linear expansion (a) of no greater than 150×10-7 K-1, and more specifically no greater than 140×10-7 K-1, and were within the desired range. In particular, the optical glasses of Examples (No. B1 to No. B11 and No. C1 to No. C12) had an average coefficient of linear expansion (α) of no greater than 110×10-7 K-1.

Further, when using the optical glasses of the Examples to form lens preforms, upon mold press molding these lens preforms, it was possible to process them into various lens forms without the occurrence of devitrification or opacity.

Above, the present invention was explained in detail with the objective of exemplification, but the present examples have only the objective of exemplification, and it is to be understood that one skilled in the art can make many modifications without departing from the concept or scope of the present invention. 

What is claimed is:
 1. An optical glass comprising, in mass %, 10.0 to 70.0% of an SiO₂ component, more than 15.0% to 31% of an Nb₂O₅ component, and 1.0 to less than 20.0% of an Na₂O component, a B₂O₃ component is no less than 7.99% and no more than 25.0%, a ZrO₂ component is more than 1.0% and less than 15.0%, a BaO component is less than 15.0%, a Bi₂O₃ component is no more than 10.0%, a ZnO component is no more than 30.0%, a Li₂O component is less than 5.0%, a mass ratio (Li₂O+Na₂O)/(ZrO₂) of no less than 1.30, and having a refractive index (nd) of 1.62 to 1.75, an Abbe number (νd) of 30 to 42, and a partial dispersion ratio (θg,F) of no greater than 0.594.
 2. An optical glass according to claim 1, wherein a mass ratio (SiO₂)/(SiO₂+B₂O₃) is no less than 0.50.
 3. An optical glass according to claim 1, wherein a mass ratio (SiO₂)/(SiO₂+B₂O₃) is no greater than 0.88.
 4. An optical glass according to claim 1, wherein, in mass %, an K₂O component is 0 to 20.0%, a TiO₂ component is 0 to 20.0%, an MgO component is 0 to 10.0%, a CaO component is 0 to 10.0%, an SrO component is 0 to 10.0%, a Ta₂O₃ component is 0 to 10.0%, an La₂O₃ component is 0 to 10.0%, a Gd₂O₃ component is 0 to 10.0%, a Y₂O₃ component is 0 to 20.0%, a Yb₂O₃ component is 0 to 10.0%, a P₂O₅ component is 0 to 10.0%, a GeO₂ component is 0 to 10.0%, an Al₂O₃ component is 0 to 15.0%, a Ga₂O₃ component is 0 to 10.0%, a WO₃ component is 0 to 10.0%, a TeO₂ component is 0 to 15.0%, an SnO₂ component is 0 to 5.0%, and an Sb₂O₃ component is 0 to 1.0%.
 5. An optical glass according to claim 1, wherein a mass sum of an Rn₂O component is 1.0% to 30.0%, wherein Rn is at least one selected from the group consisting of Li, Na, and K.
 6. An optical glass according to claim 1, wherein a mass ratio of Li₂O/Rn₂O is no less than 0.01, wherein Rn is at least one selected from the group consisting of Li, Na, and K.
 7. An optical glass according to claim 1, wherein a mass sum of an RO component is no greater than 25.0%, wherein R is at least one selected from the group consisting of Mg, Ca, Sr, and Ba.
 8. An optical glass according to claim 1, wherein a mass sum of an Ln₂O₃ component is no more than 20.0%, wherein Ln is at least one selected from the group consisting of Y, La, Gd, and Yb.
 9. A preform for use in a polishing process or precision press molding, consisting of the optical glass of claim
 1. 10. An optical element consisting of the optical glass according to claim
 1. 